(1) Field of the Invention
The present invention relates to a lens system for optical disks such as for video disks, digitally recorded audio disks, etc..
(2) Description of the Prior Art
A lens system for optical disks requires high resolving power and, therefore, it is necessary for such a lens system to keep spherical and other aberrations to a minimum. It is also necessary for the lens system to be compact and light in weight.
When such a lens system for optical disks is to be arranged by the combination of only spherical lenses, it is necessary to use at least three lenses for correcting aberrations, which adds weight and is undesirable. In order to make a lens system for optical disks light in weight and low in cost, it is desirable for such a lens system to comprise a single lens element. Even with a single lens element, the aberrations could be corrected sufficiently if both the surfaces of the lens element were formed as aspherical ones, but this would naturally involve various technical difficulties in the manufacture thereof.
The lens system disclosed in Japanese Published Unexamined Patent Application No. 17409/83 comprises a single lens element only one surface of which is aspherical and shows well-corrected aberrations. However, for this lens system to satisfy the various conditions which are required of a lens system for optical disks, the lens element must be limited to that having a refractive index in the order of 1.45 to 1.6.
The objective lens system disclosed in Japanese Published Unexamined Patent Application No. 68711/83 also comprises a single lens element having an aspherical surface, but the limitation exists in the selection of lens material.
When the workability of material and other conditions for manufacturing aspherical lenses are taken into consideration, it will not be desirable that the choice of lens material is restricted as is the case in the above mentioned prior art.